1. Field of the Invention
The present invention relates to electrophotographic copying machines, printers and like image forming apparatus, and more particularly to apparatus wherein a laser beam is used for forming images on the surface of a photosensitive member charged by a contact charger.
2. Description of the Related Art
Electrophotographic copying machines, printers or like image forming apparatus produce copy images generally by charging a photosensitive member by a charging unit, exposing the charged area to an optical image to form an electrostatic latent image, developing the latent image into a visible image, transferring the visible image to a sheet and fixing the image thereto. After the transfer, the developer remaining on the surface of the photosensitive member is removed by a cleaner.
To meet the demand that the apparatus be made more compact and less costly, various apparatus wherein the cleaner is dispensed with have also been proposed recently. For example, U.S. Pat. No. 5,148,219 discloses a so-called cleanerless image forming apparatus including a developing unit which is serviceable also as a cleaner. Such developing units have a developing sleeve or like developer support which is usually brought into contact with the surface of the photosensitive member to collect the residual developer utilizing the magnitude of field intensity of the developing region. Typical of these units are those wherein the developer support in contact with the photosensitive member develops the exposed area to form a visible image and also collects the developer remaining on the unexposed area after the transfer of the visible image to a sheet.
The voltage to be applied to the charging unit for charging the photosensitive member prior to an exposure is usually a d.c. voltage, while it is also practice to apply a voltage containing an a.c. component from the viewpoint of stabilizing the charge potential of the photosensitive member and dissipating a memory due to the residual developer (see Japanese Laid-Open Patent Application No. 56-104346).
Various image exposure units are available for different types of image forming apparatus. For example, laser beam printers and digital copying machines include a laser beam irradiator for exposing the photosensitive member to an image corresponding to an original image.
However, image noise in the form of lateral streaks which are discernible as irregularities in density appears when dot patterns or like half-tone images are formed by image forming apparatus of the type which is adapted to charge the photosensitive member by applying a charging voltage containing an a.c. component to a charging member in contact with the photosensitive member, scan the charged photosensitive member with a laser beam to form an electrostatic latent image, and develop the latent image and collect residual toner by a developing unit after transfer.